The present invention is directed to circuits and methods of driving capacitive loads, and more particularly to an improved method and BIMOS driver circuit having CMOS inverters and a PNP-NPN bipolar push-pull pair in which the speed of the output driver pair is increased maintaining good symmetry and minimizing propagation delay.
Conventional driver circuits for driving large capacitive loads, such as in a clock generator circuit for an analog to digital converter, have cascaded plural CMOS inverters I.sub.1 . . . I.sub.n, such as illustrated in FIG. 1, that are scaled by a factor .alpha. (where .alpha..congruent.e) for each additional inverter stage to increase the drive capability and to minimize circuit output rise and fall time and propagation delay. This arrangement was driven by the limited drive capability of a CMOS inverter, and consequently several inverter stages were needed to drive large loads. Each inverter, when switching between high and low states, charges and discharges its loading capacitance and draws and dumps current from and to the power supply lines. As a result, the conventional multiple stage driver consumes large amounts of real estate in an integrated circuit, and draws higher transient peak current when the driver is switching. Higher transient peaks (especially in the ground supply line) increase circuit noise.
A further disadvantage is that circuit propagation delay is directly related to the number of inverter stages and may vary widely with temperature. A uniform delay is especially useful in the design of a clock circuit that may have different loadings on different clock phases that require tracking.
With reference now to FIG. 2, BIMOS driver circuits of the prior art were improved by reducing the number of inverter stages. Some of the stages were replaced by a push-pull device having a pair of NPN-PNP bipolar transistors. For example, a driver circuit may include an input CMOS inverter 10 having a pair of CMOS devices, a push-pull bipolar transistor pair 12, and a second inverter 14 having a pair of CMOS devices. Push-pull pair 12 is used to provide a large current to drive a heavy load but cannot swing the driver circuit output rail-to-rail, and the second inverter 14 is used to provide rail-to-rail output swing after the push-pull pair 12 reaches its limit. However, when the circuit of FIG. 2 is driving a large load, the fall time is limited by the current gain of the PNP transistor Q1 in push-pull pair 12, and thus the rise and fall times become asymmetrical, where the amount of asymmetry is determined, at least in part, by the load condition. This arrangement makes matching of propagation delay difficult when multiple signal paths with different loading conditions are encountered, and may require different size components and inconsistent circuit topology for diverse applications. Further, the input signal V.sub.in is additionally loaded because it drives both first inverter 10 and second inverter 14.
A further disadvantage of the circuit of FIG. 2 is that input signal V.sub.in is inverted when it is output as V.sub.out (that is, if the state of V.sub.in is low, the state of V.sub.out is high.) A further inverter would be required to re-invert the state of V.sub.out.
Accordingly, it is an object of the present invention to provide a novel driver circuit and method for driving a capacitive load that obviates the problems of the prior art by reducing or eliminating the asymmetry caused by the PNP transistor.
It is another object of the present invention to provide a novel driver circuit and method in which the rise and fall time of the circuit are made symmetrical by feeding back driver circuit output to operate a feedback device which removes a base charge stored in the PNP transistor of a push-pull pair of PNP-NPN bipolar transistors.
It is yet another object of the present invention to provide a novel driver circuit and method in which a circuit input voltage drives a first inverter, which in turn drives a push-pull transistor pair, which in turn drives a second inverter that provides a circuit output, and where the three components are connected in parallel between a power supply and a reference voltage, so that the input voltage is not driving the second inverter.
It is still another object of the present invention to provide a novel driver circuit and method in which a first inverter, a push-pull device driven by the first inverter, and a second inverter driven by the push-pull device are connected in parallel between a power supply and a reference voltage, and in which an output from the second inverter is fed back to the base of the PNP transistor in the push-pull device to remove a base charge stored therein, thereby increasing the speed of the output from the push-pull device and reducing asymmetry in rise and fall times.
It is a further object of the present invention to provide a novel three stage driver circuit and method in which the input signal is not inverted at the output.
It is yet a further object of the present invention to provide a novel driver circuit and method in which the configuration of the circuit includes three stages regardless of application, thereby maintaining minimum propagation delay while maintaining circuit topology.
These and many other objects and advantages of the present invention will be readily apparent to one skilled in the art to which the invention pertains from a perusal of the claims, the appended drawings, and the following detailed description of the preferred embodiments.